(a) Field of Invention
The present invention relates to processes for producing batches of rubber-based composition and articles made thereof, and more particularly to processes for producing batches of rubber-based composition and shaped made thereof.
(b) Description of Prior Art
Since the creation of the rubber industry over more than a century ago, rubber materials have been essentially produced through processes based on dienes vulcanization. In the early stages, dienes-based materials were produced through plant-latex obtained from particular tropical trees such as Para rubber-plant (Hevea). Synthetic rubbers have first been introduced during the second world war to obviate a shortage in natural rubber supply. Dienes-based rubbers, either natural or synthetic, are made of macromolecular compounds exhibiting significant flowing characteristic under tensile stress. Actually, such basic materials do not present great interest for industrial applications. To prevent macromolecular slipping for the purpose of providing a material capable of sustaining deformation while recovering its initial state after stressing, blocking of the macromolecular chains to one another is required. Such molecular chains blocking is obtained through a process known as vulcanization or curing, which typically consists of cross-linking macromolecular chains at double bond sites which are present along the chains, using sulfur as the linking element or an organic peroxide. Typically, curing occurs through heating diene-based rubber materials at a temperature between about 130.degree. C. to 150.degree. C. in the presence of about 7-10% of sulfur. Mechanical properties of sulfur-cured rubbers vary with cross-linking density which is proportional to the initial sulfur concentration. In practice, compositions of various virgin (uncured) rubber types, such as natural rubber, butyl rubber, polybutadiene and neoprene, with specific additives such as curing starting agents and curing accelerators, have been developed to obtain, after curing, rubber-based material exhibiting various mechanical properties such as tensile strength, maximum tensile elongation, tear strength, embrittlement temperature and resilience.
During the past years, due to the generally high cost of uncured rubbers, reclaimed cured rubber in the form of particles or dust has been used as raw material to form a variety of manufactured articles. Essentially, the technique usually consists of mixing rubber particles with 1%-5% sulfur and curing the obtained mix in a suitable mold heated at a temperature of about 180.degree. C. while applying a pressure between 500 pound/sq.in. to 2000 pound/sq.in. with a conventional hydraulic press. Processes using such technique are generally known to be more cost effective than processes using uncured rubber as raw material, which is significantly more expensive than reclaimed rubber. However, the mechanical properties exhibited by such reclaimed materials are generally inferior, typically characterized by a maximum tensile strength of about 300 pound/sq.in., a maximum tensile elongation of about 50%, and maximum tear strength of about 100 pound/in., which properties are significantly inferior than those exhibited by uncured rubber-based compositions. Such inferior mechanical behavior is mainly due to porosity characteristics and cross-linking level. Materials made of rubber particles or dust are characterized by a porosity which is generally responsible for the appearance of microfissures under mechanical strength. Furthermore, effective contact areas between adjacent particles of a reclaimed rubber material are reduced as compared to those observed in virgin raw material. Under such conditions, cross-linking between macromolecules of adjacent particles are reduced accordingly, since most free double links of adjacent particles are not in sufficiently close proximity to be bound, and accordingly the non-reacted curing agent rapidly becomes in excess. Microfissures appearing within the rubber material initiate flaws therein which rapidly grow toward material rupture, due to the weakness of internal forces binding rubber particles together, observed at low cross-linking level.
A known technique to reduce the inherent porosity of a material made of reclaimed rubber particles consists in adding a resin, preferably a thermoplastic resin, to bind the rubber particles and therefore reduce porosity. A certain amount of uncured rubber material can also be added to increase the number of free double bond available for curing. Additives such as compatibility agents may also be added. The resultant composition is used to produce rubber materials which have been found to exhibit improved mechanical properties in the range of 400 pound/sq.in. for tensile strength, 250% for maximum tensile elongation and 180 pound/in for maximum tear strength. However, such improved reclaimed rubber materials still exhibit lower mechanical properties as compared to materials essentially made of virgin rubber, since the cross-linking level remains substantially unchanged when a thermoplastic binding resin is used, and because the electrostatic forces acting between rubber particles and thermoplastic resin are also weak. Such an improved technique is disclosed in U.S. Pat. No. 5,510,419 issued Apr. 13, 1996, to Burgoyne et al., and it teaches to produce a polymer-modified rubber composition comprising reclaimed cured styrene-butadiene rubber particles, uncured rubber, a styrene-based thermoplastic resin, a homogenizing agent, to form a blend wherein the thermoplastic resin is substantially homogeneously mixed with the cured and uncured rubbers. Additives including plasticizers, lubricants, mold release agent or viscosity modifiers such as trans-polyoctanamer rubber may also be added. Use of a batch mixer such as the well known Moriyama or Bandbury high intensity mixers is proposed to produce a moldable composition showing a temperature between 120.degree. C. and 150.degree. C., which is then transferred to a mold which is preheated at a temperature above a vulcanizing temperature of about 120.degree. C. Another similar process is disclosed in U.S. Pat. No. 5,425,904 issued Jun. 20, 1995, to Smits, which uses rubber latex to treat cured waste rubber particles with a curing agent to produce an activated moldable composition. Another similar approach is taught in U.S. Pat. No. 4,257,925 issued Mar. 24, 1981, to Freeguard, which consists of swelling reclaimed tire rubber with a monomer and then causing polymerization thereof.
In order to further improve mechanical properties of reclaimed rubber materials, the addition of various reactive cross-linking resin binders to the curing agent has been proposed. In U.S. Pat. No. 3,489,710 issued Jan. 13, 1970, to Bonotto et al., ethylene-based flexible resins reactive with a curing agent such as sulfur are mixed with reclaimed rubber particles in the presence of the curing agent using a high intensity batch mixer such as a Bandbury mixer. Similarly, in U.S. Pat. No. 4,481,335 issued Nov. 6, 1984 to Stark, a liquid sulfur-curable polymeric binder, namely a homopolymer or copolymer of 1,4-butadiene and substituted butadiene, is blended with cured rubber scrap and sulfur to produce a treated rubber material which can be used in large proportion as a filler or extender for uncured rubbers. Although known prior art processes employing reclaimed rubber have heretofore proved to be capable of producing, at lesser cost, shaped rubber-based products of various quality in terms of mechanical properties, in order to be practiced on an industrial basis, these processes generally require the use of large and expensive high intensity mixing equipment, generally limited to long production cycles to provide a composition homogeneity which is required to obtain uniformity of physical properties, which requirement may affect the effective productivity of the process. Furthermore, known processes for mixing curable rubber-based compositions may not provide proper reduction of humidity within a batch to be formed, which may cause dangerous high pressure steam discharge upon removal from the mold. Therefore, there is still a need for a process for producing batches of rubber-based composition and articles made therefrom which overcome the foregoing drawbacks of the prior art.